1. Field of the Invention
This invention relates to a computed tomography scanner that is adapted, for example, to non-destructive examination devices.
2. Description of the Related Art
Computed tomography scanners of this kind (hereinbelow abbreviated to CTs) include second-generation CTs of, for example, the traverse/rotation (hereinbelow abbreviated to T/R) system shown in FIG. 9 and third-generation CTs of the rotate/rotate system (hereinbelow abbreviated to R/R) shown in FIG. 10.
In a second-generation CT of the T/R system shown in FIG. 9, an X-ray fan beam 4 of fan angle .alpha. that is output from an X-ray source 1 is directed on to a subject to be examined not shown, placed on a T/R table 3 corresponding to the scanning area. Meanwhile, T/R table 3, on which the subject is placed, is rotated as shown by an arrow R and is traversed as shown by an arrow T, thereby effecting scanning. The X-rays that have passed through the subject are detected by a detector 2. This X-ray transmission data of the subject that is thus detected is collected, and this collected X-ray transmission data of the subject is subjected to processing by a reconstruction device, not shown, to obtain a tomographic image of the subject. With T/R table 3, the necessary scanning area is guaranteed by the space between X-ray source 1 and detector 2. From the point of view of efficiency of utilization of the X-rays, a distance SDD between X-ray source 1 and detector 2 should be very small.
Hereinafter scanning will be described in more detail. A prescribed distance traverse operation as shown by arrow T in FIG. 9 is performed such that T/R table 3 is made to traverse through the fan angle .alpha. of the X-ray beam. Indexing of the angle of rotation of T/R table 3 is performed with fan angle .alpha., and data collection is performed by effecting rotation through an angle .alpha. for each traverse.
The prior art second-generation CT of the T/R system described above is disclosed in detail in U.S. Pat. No. 5,027,378 issued on Jun 25, 1991.
However, when a subject is placed on T/R table 3, if some parts of the subject stick out from T/R table 3, they interfere with, for example, the X-ray shutter, the collimator and the frame etc, not shown, on X-ray source 1 side, or interfere with structural objects, not shown, on detector 2 side, with the result that scanning cannot be achieved. In general, T/R table 3 would be capable of 360.degree. rotation.
In the case of the third-generation CT of the R/R system shown in FIG. 10, the arrangement of X-ray source 1 and detector 2 is the same as in FIG. 9. In this system, instead of both traverse and rotation only rotation can be performed, and the scanning action is achieved solely by rotation of a R table 5, by rotating R table 5 as shown by arrow R.
Also, when a subject is placed on R table 5, just as in the case of the T/R system described above, if the subject is placed on R table 5 with some parts sticking out, such parts of the subject interfere with structural items, not shown around the periphery of X-ray source 1 or detector 2, preventing scanning. Also, in general, R table 5 can be rotated through 360.degree..
As described above, in the case of the prior art CTs as shown in FIG. 9 and FIG. 10, if the subject has external dimensions larger than the scanning area, there is the problem that scanning cannot be achieved.